No Load Test of

The impedance of magnetizing path of induction motor is large enough to obstruct flow of current. Therefore, small current is applied to the machine due to which there is a fall in the - impedance value and rated is applied across the magnetizing branch. But the drop in stator- impedance value and power dissipated due to stator resistance are very small in comparison to applied voltage. Therefore, there values are neglected and it is assumed that total power drawn is converted into core loss. The air gap in magnetizing branch in an induction motor slowly increases the exciting current and the no load stator I2R loss can be recognized. One should keep in mind that current should not exceed its rated value otherwise accelerates beyond its limit. The test is performed at poly-phase and rated frequency applied to the stator terminals. When motor runs for some times and bearings get lubricated fully, at that time readings of applied voltage, input current and input power are taken. To calculate the rotational loss, subtract the stator I2R losses from the input power.

Calculation of No Load Test of Induction Motor

Let the total input power supplied to induction motor be W0 watts.

Where,

V1 = line voltage

I0 = No load input current

Rotational loss = W0 – S1 Where,

S1 = stator winding loss = Nph I2 R1

Nph = Number phase The various losses like windage loss, core loss, and rotational loss are fixed losses which can be calculated by

Stator winding loss = 3Io2R1 Where,

I0 = No load input current

R1 = Resistance of the motor

Core loss = 3GoV2

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Blocked Rotor Test of Induction Motor

The induction motors are widely used in the industries and consume maximum power. To improve its performance characteristics certain tests have been designed like no-load test and block rotor test, etc. A blocked rotor test is normally performed on an induction motor to find out the leakage impedance. Apart from it, other parameters such as torque, motor, short-circuit current at normal voltage, and many more could be found from this test. Blocked rotor test is analogous to the short circuit test of transformer. Here shaft of the motor is clamped i.e. blocked so it cannot move and rotor winding is short circuited. In motor rotor winding is short circuited through slip rings and in cage motors, rotors bars are permanently short circuited. The testing of the induction motor is a little bit complex as the resultant value of leakage impedance may get affected by rotor position, rotor frequency and by magnetic dispersion of the leakage flux path. These effects could be minimized by conducting a block rotor current test on squirrel-cage rotors. Process of Testing of Blocked Rotor Test of Induction Motor In the blocked rotor test, it should be kept in mind that the applied voltage on the stator terminals should be low otherwise normal voltage could damage the winding of the stator. In block rotor test, the low voltage is applied so that the rotor does not rotate and its speed becomes zero and full load current passes through the stator winding. The slip is unity related to zero speed of rotor hence the load resistance becomes zero. Now, slowly increase the voltage in the stator winding so that current reaches to its rated value. At this point, note down the readings of the voltmeter, wattmeter and ammeter to know the values of voltage, power and current. The test can be repeated at different stator voltages for the accurate value.

Calculations of Blocked Rotor Test of Induction Motor

Resistance and Leakage Reactance Values

In blocked rotor test, core loss is very low due to the supply of low voltage and frictional loss is also negligible as rotor is stationary, but stator cupper losses and the rotor cupper losses are reasonably high.

Let us take denote copper loss by Wcu. Therefore,

Where, Wc = core loss

Where, R01 = Motor winding of stator and rotor as per phase referred to stator.

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Thus, Now let us consider

Is = short circuit current

Vs = short circuit voltage

Z0 = short circuit impedance as referred to stator

Therefore,

X01 = Motor leakage reactance per phase referred to stator can be calculated as

Stator reactance X1 and rotor reactance per phase referred to stator X2 are normally assumed equal. Therefore,

Similarly, stator resistance per phase R1 and rotor resistance per phase referred to stator R2 can be calculated as follows:

First some suitable test are done on stator windings to find the value of R1 and then to find R2 subtract the R1 from R01

Short Circuit Current for Normal Supply Voltage

To calculate short circuit current Isc at normal voltage V of the stator, we must note short-circuit current Is and low voltage Vs applied to the stator winding.

SNSCE/EEE/EM-II/UNIT-5/P.SANGEETHA